THE TEST BAN treaty is an imperfect instrument of pressure and persuasion and at best is more likely to slow than stop, let alone reverse, the proliferation of nuclear arms. It would constrain, but only uncertainly, their acquisition by states and non-state parties that do not currently possess them, and their modernization by those that do. Test ban critics have had decades to hone their arguments. As the Republican leadership of the Senate -- believing the moment is ripe to kill it -- suddenly takes up ratification of the treaty, the Clinton administration is struggling for the requisite two-thirds approval.

Still, the administration is on the right track. For the real issue posed by this treaty is not whether it serves well in every particular -- it is too late for that. The issue is whether it improves on the situation that now obtains and on the situation that would develop in the event of the repudiation or the indefinite shelving of the treaty. We believe the treaty offers such an improvement. It opens a way to block at least some proliferation of new and improved nuclear weapons into various hands. The treaty will still be years in coming into effect; in those years the United States can exert a more powerful influence if it has itself ratified.

No, some say, the game is already over: The technology is loose and available, and the security anxieties and political ambitions that drive proliferation are rampant. But this becomes a self-fulfilling prophecy, a form of political surrender. Security anxieties can be addressed by means other than proscribed weapons. Political ambitions can be curbed and sublimated. Considerations of economic consequence can be brought to bear. Nuclear programs already underway can be contained. Why would the United States, whose global interests make it more in need of a safe international environment than any other country, deny itself a diplomatic device of evident potential benefit?

Others will ignore the treaty or cheat on it: That is the skeptics' second large objection. It's true: These things will happen. But the treaty adds substantially to the monitoring capabilities available for detection of bomb-building or cheating. And even if some of the desired detection fails, it remains to be proven that the cheating actually matters strategically. Meanwhile, the treaty puts an international imprimatur on the United States' immensely superior capacity to check the reliability of its nuclear stockpile and conduct the non-explosive computer and lab tests that permit the United States to upgrade. Less nuclear capability for them, more for us: How can Americans spurn a bargain so transparently and even one-sidedly to the American advantage?

President Clinton has a plain political interest in seeing to the ratification of a treaty that he himself signed; it is part of his "legacy." But it is no less so that the treaty can be made to serve the surpassing American interest in limiting the spread of weapons of mass destruction. The opponents, after making their expert critiques, finally have nothing -- no diplomatic instrument or policy -- to put in its place. In those circumstances, to block or sidetrack this 50-year international project is to send a terse and unmistakable signal to would-be proliferators everywhere, starting in South Asia. The treaty may be imperfect, but it's necessary for American leadership and security.